Electronic devices use phase-locked loops to manipulate electronic signals. A phase-locked loop (PLL) circuit is a control system that generates an output signal having a phase and frequency that is related to (e.g. identical or proportional to) the phase and frequency of an input signal, which is often referred to as a reference signal. The PLL circuit compares the phase of the input signal with the phase of the signal at the output of the PLL circuit and adjusts the frequency of an oscillator (e.g. a voltage-controlled oscillator), included in the PLL circuit, to keep the phase of the generated output signal matched to the phase of the input signal. By keeping the phases of these signals locked to each other, it can be assured that the frequency of the output signal also matches the frequency of the input signal or an integer multiple of the frequency of the input signal.
Keeping the input and output signals locked to the same frequency allows for signal processing and communication to occur at higher speeds. PLL circuits are widely employed in radio, telecommunications, computers, and other electronic applications. In various applications, PLL circuits may be used to recover a signal from a noisy communication channel, generate stable frequencies at a multiple of an input frequency (frequency synthesis), or distribute clock timing pulses in digital logic designs such as microprocessors. Since a single integrated circuit can provide a complete PLL circuit, the technique is used in modern electronic devices, with output frequencies from a fraction of a hertz up to many gigahertz.
As PLL circuits are used in devices having faster speed requirements and having lower power supply voltages, electronic noise (e.g. thermal noise) and device size become significant factors in the design of the PLL circuit.